Segregations and Precipitations in a Heat treated Al-Zn-Mg Alloy

Abstract

Segregation and precipitation of a locally prepared Al - 10.0 wt% Zn - 2.5 wt% Mg alloy was studied using scanning electron microscope and electron probe microanalyser. Samples were solution treated at various temperatures ranging from 450 to 550°C for 3h. Samples solution treated at 480°C for 3h were aged at 130 and 300°C from 1 to 72h. In the as cast alloy segregation of Mg and Zn at grain boundaries and liquid droplets within the grains was observed showing cellular/lamellar structure at higher magnification. Samples solution treated above 500°C have very sharp dendritic structure. Microhardness of the segregated areas was much higher than that of the normal matrix Samples aged at 130°C show initial increase in hardness with increase in ageing time up to 24h after which hardness becomes constant. Hardness of samples aged at 300°C decreases with increase of ageing time up to 7h which may be due to formation of incoherent MgZn2 precipitates. After 7h hardness remains constant.